In a mobile communication environment, the amplitude or phase varies due to Rayleigh fading associated with movement of the relative locations of a mobile station and a ground station. According to the phase modulation method for transmitting information using a carrier phase, differential coding is generally used to load information on relative phases of a preceding and a following symbols, and a receiver carries out delayed detection to identify and determine the information data. This delayed detection, however, differentially codes the transmitted data as described above, whereby one-bit error within a wireless section corresponds to a two-bit error in the information data. Thus, at the same signal power to interference/noise power ratio (SNIR), the two-phase phase modulation method (BPSK modulation) has a higher reception error rate than synchronous detection by 3 dB.
In addition, absolute synchronous detection that identifies and determines an absolute phase of a received signal for each symbol has an efficient reception characteristic, but it is difficult to determine the absolute phase in a Rayleigh fading environment.
To solve this problem, a method has been proposed which inserts pilot symbols between data symbols so as to use these pilot symbols to estimate a channel for the data symbols. One of the methods for inserting pilot symbols, for example, time-multiplexes data symbols and pilot symbols into one channel (time multiplexing method; FIG. 16). Documents 1 to 3, which will be cited below, propose channel estimation methods using this time multiplexing method.
The document 1 (Electronic Information Communication Society Journal Vol. J72-B-11, No. 1, pp. 7 to 15, January 1989, SANPEI “Land Mobile Communication 16QAM Fading Distortion Compensation”) proposes a method for solving the above problem by estimating and compensating for fading distortion using pilot symbols inserted between data symbols (information symbols) at a fixed cycle and the phases of which are known. This method inserts the pilot symbols into a communication channel at the rate of one pilot symbol per several data symbols to estimate a transmission path based on received phases of the pilot symbols. Signals received during each pass of each transmitter are measured at pilot symbols before and after a desired data symbol section for amplitude and phase, and the measured values are interpolated to estimate and compensate for transmission path variations within the data symbol section.
Document 2 (Electronic Information Communication Society Technical Report RCS97-74, ANDO at el. “RAKE Reception Using the Multislot Weighted Averaging Channel Estimation Method with Pilot Symbols in DS-CDMA”) proposes a method for carrying out more accurate channel estimation using more pilot symbols. A channel for data symbols is estimated using pilot symbols inserted between data symbols at a fixed cycle. Specifically, pilot symbols (estimated complex fading envelope) in a plurality of slots before and after a slot for which a channel is estimated are averaged (in-phase addition), and the average value is subjected to weighted averaging using a weighting factor to obtain a channel estimated value. The channel estimation accuracy is thereby improved to prevent thermal noise or multipass interference and interference from other stations.
Document 3 (Electronic Information Communication Society Technical Report RCS98-20, ABETA at el. “Characteristics of the DC-CDMA Adaptive Plural Symbol Weighted Averaging Pilot Channel Transmission Path Estimatiod Method”) proposes a method of adaptively controlling a weighting factor to reduce the effects of thermal noise while improving the capability of following fading variations. According to this method, channel estimation involves weighted averaging, and this weighting factor is sequentially varied using an adaptive signal to determine an optimal weighting factor.
The pilot symbol insertion methods include not only the time multiplexing method but also a parallel time multiplexing method (FIG. 1) and a parallel method (FIG. 22) that time-multiplex pilot symbols into a control channel parallel-multiplexed for a data channel.
For the parallel time multiplexing method, it is desirable to execute accurate channel estimation by subjecting the pilot symbols to weighted averaging to calculate a channel estimated value for the data symbols in the data channel.
In addition, according to the methods in Documents 1 to 3, channel variations within each slot are assumed to be small, and the same pilot symbol is used for all the data symbols within one slot to obtain the same channel estimated value. Consequently, the characteristics are disadvantageously degraded during fast fading.
Further, the method in the Document 2 provides a fixed weighting factor, and when the weighting factor for slots temporally remote from a desired slot is increased to reduce the effects of thermal noise, the capability of following fading variations is disadvantageously degraded, thereby causing the channel estimation accuracy to be degraded. Another problem of the method in the Document 3 is that despite the solution of the problem of the Document 2, the use of the adaptive signal process may make the configuration of a demodulation device complicated.
In the mobile communication environment, the amplitude or phase varies due to Rayleigh fading associated with movement of the relative locations of a mobile station and a ground station. The synchronous detection process using pilot signals is known as a method for compensating for the variations of the amplitude or phase to effectively synthesize multiple passes.
According to this method, a transmitter transmits a known pilot signal, while a receiver demodulates and temporally averages this pilot signal to estimate a channel. Then, the estimated channel vector is used to correct a phase of a data signal, which is then subjected to RAKE synthesis, thereby achieving demodulation using power of the received signal.
Since the channel estimation accuracy directly affects data quality, averaging must be carried out using appropriate temporal sections and an appropriate weight sequence. One sequence that improves the channel estimation accuracy is conventionally used as the weight sequence.
When the receiver estimates a channel, the channel estimation accuracy can be improved to enable high-quality communication, by using an appropriate weight sequence to average pilot signals. The appropriate weight sequence, however, depends on propagation conditions, principally, the movement speed, as described above.
That is, at a lower movement speed, channel variations occur at a lower speed, so that a weight sequence that increases the averaging time is effective, whereas at a higher movement speed, fast channel variations must be followed, so that a weight sequence that reduces the averaging time to some degree is effective.
However, the known channel estimation method using only the one weight sequence fails to enable averaging suitable for every movement speed, resulting in degradation of communication quality, an increase in transmission power required, a decrease in communication capacity achieved.
In addition, methods for varying the weight sequence depending on the movement speed include a method of detecting the movement speed to vary the weight sequence depending on the detected speed. A problem of this method, however, is that if the speed detection accuracy or the detection following capability is degraded, improvement of communication quality, a reduction in transmission power required, and an increase in capacity cannot be realized.